1470
J . Phys. Chem. 1986, 90, 1470-1478
measured at ionic strengths ranging from 0 to 1.0 and plotted as shown in Figure 6 . The theoretical curve calculated with eq 11 with ZAZB= 8 and a = 1 .O is shown by the broken curve. The solid curve is obtained by the parameter fitting for k and a. The value obtained for a is 2.9 which gives an r of 8.8 t f . This value of intermolecular distance seems reasonable according to the molecular size of ZnTMPyP4+ and viologen group. Conclusion
The following features for photoinduced electron transfer from cationic zinc porphyrin to the polymer viologen were obtained. (1) Quantum yields ai for the cation radical formation for polymer viologen are higher than that for MV2+. (2) Decreasing viologen-viologen distance in the polymer does not increase ai. (3) Ionic pendant group decreases the reduction potential of viologen group, but does not increase ai. (4) For the polymers used, ai decreases monotonously with
increasing ionic strength, while that for MV2+has a maximum value. (5) The high rate of viologen radical formation for polymer viologen is due to the decrease of the reaction rate of back electron transfer. (6) For the reaction between cationic zinc porphyrin and pendant viologen group in the polymer, the Debye-Huckel relationship is applicable.
Acknowledgment. We are grateful to Mr. K. Yamaguchi for his participation in the laser flash measurements. This study was supported partly by a Grant-in-Aid for Scientific Research from the Ministry of Science, Education and Culture. Registry No. PV4(5), 100351-45-9; PV4(5+50), 100351-46-0; PV4(5+), 100351-47-1;PV4(5-), 100351-48-2;AQN, 75855-87-7; AQN(+), 100367-06-4; MV2+,4685-14-7; MV*., 25239-55-8; TEOA, 102-71-6; ZnTMPyP4', 40603-58-5; ZnTTMPyP', 83294-32-0; ZnTPP, 1407480-7; ZnTSPP", 80004-36-0.
Water Dynamics in Aqueous Electrolyte Solutions from Proton, Deuterium, and Oxygen-I7 Nuclear Magnetic Relaxation J. R. C. van der Maarel, D. Lankhorst, J. de Bleijser, and J. C. Leyte* University of Leiden, Gorlaeus Laboratories, Department of Physical and Macromolecular Chemistry, 2300 R A Leiden, The Netherlands (Received: July 31, 1985; In Final Form: October 29, 1985)
The relaxation rates of 2H and 170and the rate of 'H due to 170were obtained in a series of aqueous electrolyte solutions. From these data conclusions are reached about the influence of the ions on the dynamical behavior, the O-H bond distance, and the 2H coupling constant of neighboring water molecules. It is found that the changes in the 0-H distance and the 2H coupling constant induced by most ions investigated are moderate. The dynamical behavior of the water molecules is changed significantly as shown by the correlation times and the emergence of moderate motional anisotropy. The concentration dependence of these effects will be discussed.
Introduction
In an attempt to isolate the influence of ions on the properties of neighboring water molecules from nuclear relaxation data on electrolyte solutions several problems have to be dealt with. To begin with, a breakdown of the overall rates in terms of contributing "states" (i.e., environment) of the water molecules must be defined and the division of the electrolyte effect into contributions of the ionic species should be made in accordance with the results of other experimental techniques. Next, the influence of an ion on the molecular properties such as the intramolecular field gradients and the 0-H bond distance have to be considered. Then a dynamical model for the water molecules in the hydration phase has to be introduced. In N M R such a model specifies the reorientation of the water molecules with respect to a laboratory fixed coordinate system, yielding, e.g., one or more components of a molecule fixed diffusion tensor. Therefore the last and probably most difficult question to resolve is the orientation of the diffusion tensor with respect to the neighboring ion. In the work reported here some of these problems have been dealt with and for the last-mentioned question our data suggest a reasonable answer for at least the cations. In this Introduction the problems just discussed are analyzed in a general manner leading to choices and solutions, which will be applied in the discussion of our results. To describe the relaxation behavior of water nuclei in a salt solution a three-state model is introduced. Water molecules in the direct neighborhood of an ion are defined to be hydration water, thus forming the hydration shell. The remaining part is called bulk water. This concept only makes sense when the 0022-3654/86/2090- 1470$01.50/0
residence times in the states are longer than the motional time scale within the states. With this model the motion within the hydration shell turns out to be in the picosecond range. The available residence times will be discussed below.] For ions with unpaired electrons it is possible to obtain the residence time of a water molecule in the hydration shell from the effect of these electrons on the H or 170relaxation. This residence time appears to be in the range of microseconds. For diamagnetic ions this method cannot be applied. For these ions the residence time of a water molecule in the hydration shell will be shorter as a consequence of the lack of coordinating d-electron orbitals. Experimental evidence for these ions is scarce. The residence times of water molecules in the hydration shells of Mg2+ and Li+ have been determined by N M R methods. Neutron scattering experiments provide a means to determine the translational correlation time of pure water. These results will be briefly discussed and compared to some M D results. The residence time of water molecules in the hydration shell of Mg2+is 10" s (25 "C). This value was obtained by line-width measurements of oxygen-17 in a Mg(ClO,), solution, while the bulk water resonance was broadened by addition of paramagnetic Mn2+.2 Fabricand and Goldberg examined the proton relaxation in a 7LiC1 and a 6LiC1 ~ o l u t i o n . ~The 'Li nucleus has a larger nuclear magnetic moment than 6Li, therefore by a careful comparison of proton relaxation rates in these two isotopic solutions it is possible to obtain information about the Li(H20),,+ complex. (1) J. P. Hunt and H. L. Friedman, Prog. Inorg. Chem., 30, 359 (1982). (2) J. W. Neely and R. E. Connick, J . Am. Chem. Soc., 92, 3476 (1970). (3) B. P. Fabricand and S. S. Goldberg, Mol. Phys., 13, 323 (1967).
0 1986 American Chemical Society
The Journal of Physical Chemistry, Vol, 90, No. 7, 1986 1471
Water Dynamics in Aqueous Electrolyte Solutions TABLE I: Residence Time of Water Molecules in the Hydration Shell Obtained from MD Simulations and Some Experimental Results, as well as the Translational Correlation Time of Pure Water ion T, K T, ps ion T,K T, ps Mg2' Li+
N a+ Kt
298 278 298 282 274
F 33.3" 146 9.9" 4.8"
c1H20
H20
278 287 286 298
20.3' 4.5' 4.5" 1 .7c
" Reference 6. Reference 1 . M.-C. Bellissent-Funel,R. Kahn, A. J. Dianoux, M. P. Fontana, G . Maisano, P. Migliardo, and F. Wanderlingh, Mol. Phys., 52, 1479 (1984). dReference 2. However, Fabricand and Goldberg were not able to explain their results by a rigid, long-lived Li(H20)n+complex. Hertz reinterpreted these results, taking into account an internal motion of a hydration molecule about the Li-0 axis, but he also concluded that the hydration shell of Li+ is not rigid and not long lived.4 Finally, using the neutron diffraction results of Newsome et al.,5 Hunt and Friedman repeated the calculation of Hertz. They obtained a residence time of 14 ps, assuming a rotational correlation time of 30 ps for the whole Li(H20)4+complex.' Molecular dynamics simulations also provide information about these residence times. Impey et al., using an analytical potential energy function of Matsuoka, Clementi, and Yoshimine (MCY), obtained a value of the residence time of water molecules within the first hydration shelL6 During the M D run they noted the number nion(t),i.e. the number of molecules which lie initially within the first hydration shell and are still there after a time t has elapsed. Except at short times, this number decays exponentially. These residence times together with the experimental residence times of water molecules in the hydration complexes of Li+ and Mgz+ as well as the translational correlation time of pure water are presented in Table I. The translational correlation time of pure water and the lifetime of the hydration shell of Li' as given by the MD results are a factor 2.5 longer than the corresponding N M R and neutron scattering values. The residence time of water molecules in the hydration shell of monovalent ions increases from about 2 ps (obtained by dividing the M D result by the factor 2.5) to about 14 ps in the case of Lit. From the work reported here reorientational correlation times within the hydration shell turn out to increase from 1 ps in case of I- to about 5 ps when the solute is Li+. The residence times of water molecules in the hydration shells of Mg2+, Ca2+,Li+, Na+, and F are certainly longer than the motional time scale within the shell, but this condition is not with certainty met in the cases of K+, Cs+, C1-, Br-, and I-. The number of water molecules within a hydration shell n* is also an unknown quantity. Neutron scattering experiments provide detailed information about the structure of electrolyte solutions. Hydration numbers obtained from the ion-water correlation functions are presented in Table 11. An additional source of information consists of the results of M D simulations. The average number of hydration water molecules counted during the M D run is also presented in Table 11. To investigate the dynamics of a solution, different intramolecularly determined hydration water relaxation rates will be compared. These rates are calculated with equal hydration numbers for all ions. The conclusions appear not to be very sensitive to a particular choice of the hydration number within the range of the values presented in Table 11. A value of six will be chosen for all ionic species. A homogeneous distribution of deuterium between hydration water and the bulk is assumed. This has been shown to be the case for C1-, Br-, and I-.7 (4) H. G. Hertz, Mol. Phys., 14, 291 (1968). (5) J. R. Newsome, M. Sandstrom, G. W. Neilson, and J. E. Enderby, Chem. Phys. Lett., 82, 399 (1981). ( 6 ) R. W. Impey, P. A. Madden, and I. R. McDonald, J . Phys. Chem., 87, 5071 (1983): (7) 0. A. El Seoud, M. I. El Seoud, P. A. R. Pires, and J. P. S. Farah, J . Phys. Chem., 88, 2269 (1984). M. I. El Seoud and 0. A. El Seoud, Ber. Bunsenges. Phys. Chem., 88, 742 (1984).
TABLE II: Hydration Numbers from Neutron Diffraction Experiments and MD Simulations
ion Li+
solute LiCl
Nat
NaCl
K+ Ni2+
CaZt
KCI NiClz
molality 3.57
278 293.5 282 274 293.5
1.O
1.o
4.41 3.05 1.46 0.85 0.46 0.086 4.49
CaC1,
F
c1-
T, K
278
LiCl
3.57 287
n'
5.5 f 0.3O 5.3b 8' 6.0b 1.5b SC 5.8 f 0.2" 5.8 f 0.2" 5.8 f 0.2O 6.6 f 0.5" 6.8 0.8" 6.8 f 0.8" 5.5 f 0.2" 5.Sb 5.9 f 0.2" 7.2b
*
'Reference 26. bReference6. cN. Ohtomo and K. Arakawa Bull. Chem. SOC.Jpn., 53, 1789 (1980). According to the model used here each experimental relaxation rate is expressed as a weighted sum of relaxation rates in the three states. In eq 1 all relaxation rates are understood to be corrected
for small solvent isotope effects. This is explicitly indicated by a superscript c for RC,only. In this equationf' is the mole fraction of hydration water. The relaxation rate of bulk water is denoted by R: and the relaxation rate of cation or anion hydration water is represented by R: or R;, respectively. The subscript x can be 0, D, or OH when the relaxation path is respectively oxygen-17 quadrupolar, deuterium quadrupolar, or proton-oxygen-17 dipolar interaction. If we define a hydration number nil the mole fraction f' may be expressed in terms of the molality m of the solution:
f' = n * m / 5 5 . 5
(2)
Equation 1 is valid when the residence time of water molecules in the defined states is shorter than the macroscopic relaxation times and longer than the motional time scale within the states. The former condition is easily satisfied, because the shortest s. The latter condition has just relaxation time is about 5 X been discussed. The residence time of water molecules in the hydration shell of K+, Cs+, Cl-, Br-, and I- approximates the motional time scale within the shell, but even in this case eq 1 is still a good approximation.8 As a consequence of the electric neutrality it is impossible to separate the cationic and anionic influence on the relaxation rates. Neutron scattering experiments on different chlorides show that C1- hydration is independent of type of cation and of ionic ~ t r e n g t h . ~ 'These '~ experiments show the hydration of K+ to be very weak." Therefore it is possible to study the influence of cations by comparing different chlorides and for the anions potassium salts may be used. To calculate the hydration water relaxation rates R: or R; the contribution of the C1- or K+ ion will be assumed zero, but for any small constant value the resulting trend will be the same. Under these conditions eq 1 reduces to
R: = (1 -p)RP +PR:
(3)
The relaxation rates in the hydration spheres may now be calculated after evaluation of the relaxation rate RP in bulk water. Quasielastic neutron scattering experiments of Hewish et a1.I2 show (8) H. G. Hertz, Water, Franks, F., Ed., Vol. 3, Plenum R e s : New York, 1973. (9) S. Cummings, J. E. Enderby, G. W. Neilson, J. R. Newsome, R. A. Howe, W. S. Howells, and A. K. Soper, Nature (London), 287, 714 (1980). (10) S. Biggin, J. E. Enderby, R.L. Hahn, and A. H. Narten, J . Phys. Chem., 88, 3634 (1984). (1 1) G. W. Neilson and N. Skipper, Chem. Phys. Lett., 114, 35 (1985).
1412 The Journal of Physical Chemistry, Vol. 90, No. 7, 1986
a small influence on water molecules beyond the first hydration shell of Ni2+and Mg2+. An influence on water molecules beyond the first hydration shell of Li+ and Cs', which both have a smaller electric field strength at the surface, could not be detected. Therefore the relaxation rate of bulk water will be assumed to be equal to the relaxation rate of pure water. Under extreme narrowing conditions the hydration water relaxation rate R: is the product of an interaction constant and a dynamical effective correlation time 7:: R: =
c7:
(4)
The interaction constant of the H-170 dipolar interaction is given by
Here rgH denotes the 0-H bond length of a hydration water molecule. The interaction constants of the quadrupolar interaction are
when the relaxing nucleus is D and
when it is I7O. To enable a discussion of the dynamical features of hydration water the coupling constants should be known, thus allowing the calculation of the effective correlation times from the experimental values of R:. Although the coupling constants are known reasonably well in bulk water, their values for water molecules in close proximity to an ion is unknown. This problem may be solved in a manner similar to the determination of the coupling constants in pure water.13 This possibility arises due to the fact that some of the effective correlation times are in fact equal. The effective correlation times r: characterize the orientational correlation loss of a particular interaction tensor for each nucleus studied. This can be caused by reorientational motion of water molecules within the first hydration shell or by an overall reorientational diffusion process of the whole ion-hydration water complex. The correlation time of the latter process will be approximately 30 ps, while the experimental correlation times turn out to be a few picoseconds. Because the time scale of this overall diffusion process is much longer than the experimental correlation times this process will be neglected. The lattice part of the coupling Hamiltonian, i.e. the interaction tensor, will be described by the irreducible elements Q2).In the principal axis system of the dipolar interaction tensor, one has Q2)= 60k(2/3)'/2roH-3.In the rincipal axis system of the = eq(3/2)'/', V!!! =i -77/2eq, quadrupolar interaction, one has with the asymmetry parameter 7 being defined as 7 = (Vyy-
vxx)/vz,.14
The orientation of the principal axis system of the H-I7O dipolar interaction tensor coincides with the orientation of the principal axis system of the D quadrupolar interaction tensor. The asymmetry parameter of the D field gradient is very small ( q D= 0.135, gas value). As a consequence the V !:ifield gradient component is small with respect to the dominant component. A combination of these two features results in an equality of the experimental correlation times 7; and 7gH within experimental accuracy and this equality is independent of the symmetry of the
6')
(12) N. A. Hewish, J. E. Enderby, and W. S. Howells, J. Phys. C,16, 1777
van der Maarel et al. diffusion tensor (one has to take a small isotope effect into account; this will be discussed later). Therefore if the correlation times 7; and 7gH as determined from the relaxation rates and the application of eq 5 with the use of molecular properties of bulk water molecules are not equal, this would indicate that c",and C$, change when the water molecule becomes a member of the hydration shell. Rephrased in concrete experimental terms this means that a change in the ratio R;/RgH with respect to its pure water value is a direct indication of a change in the values of rfOHand C$ upon transfer of a water molecule from the bulk to a hydration shell. As quantum chemical and empirical relations connecting c",and rfOHare known it is possible to calculate these quantities from the observed values of R i and RgH as will be shown in the next section. For most ions considered here the effects are small and the dynamical behavior of hydration water will be discussed by using effective correlation times as calculated from the relaxation rates assuming bulk water coupling constants. The motion within the hydration shell will be represented by a rotational diffusion process having axial symmetry. The reader is referred to ref 15. For a study of the intramolecularly determined relaxation paths and the relative relaxation behavior as a function of the diffusion tensor properties, ref 16 may be consulted. The diffusion tensor is characterized by two components Dlland D , and by the orientation of the principal axis system of this tensor with respect to the principal axis system of the interaction tensor. Under extreme narrowing conditions the effective correlation time will be given by
with 7,-'
= 60,
+ m2(D,,- D l )
(7)
The Wigner matrix elements DZi describe the transformation of the principal axis system of the interaction tensor to the principal axis system of the diffusion tensor, characterized by the orientation Qr
Experimental Section
Chemicals and Solutions. Oxygen isotopically enriched water was obtained from Monsanto Research Corp., Miamisburg, containing 9.9% 0-16, 51.1% 0-17, and 39.0% 0-18. D 2 0 was obtained from E. Merck, Darmstadt. Distilled water was deionized and filtered by a Milli-Q water purification system (Millipore Corp.). MgC12, analytical reagent, was obtained from Baker; the remaining salts are Suprapur quality from Merck. The solutions were prepared by weight in a cold room at 5 "C to minimize exchange with atmospheric humidity. The solutions prepared for proton measurements were shaken five times with argon to remove gaseous oxygen and relaxation rates were measured within 2 h. N M R tubes (Wilmad 10 mm) were heated in a NaHCO, solution, heated in an EDTA solution, and then stored for at least 1 week filled with deionized and filtered water. Before drying the conductance of the contents was checked; it never exceeded 2X 0-' cm-I. For proton measurements of KF solutions, quartz sample tubes were used. Determination of Relaxation Rates. Solutions of CsCl, KC1, KI, and KBr were measured on a home-modified Bruker SXP spectrometer equipped with a 6.3-T superconducting magnet (Oxford Instruments). The temperature was maintained at 298 & 0.5 K with a variable-temperature unit (Bruker B-VT 1000). Solutions of MgC12, CaCl,, LiCl, and NaCl were measured on a home-built spectrometer equipped with a 2.1-T electromagnet (Bruker). The temperature was maintained at 25.0 & 0.1 O C by
( 1983).
(13) D. Lankhorst, J. Schriever, and J. C. Leyte, Ber. Bunsenges. Phys. Chem., 86, 215 (1982). (14) H . W. Spiess, N.M.R., Basic Principles and Progress, Diehl, Ed., Vol. 15, Springer, Berlin, 1978.
(15) W. T. Huntress, Adu. Magn. Reson., 4, 1 (1970). (16) C. W. R. Mulder, J. Schriever, and J. C. Leyte, J . Phys. Chem., 87, 2336 (1983).
The Journal of Physical Chemistry, Vol. 90, No. 7, 1986 1473
Water Dynamics in Aqueous Electrolyte Solutions
TABLE III: Isotope Effect Corrected Relaxation Rates and Isotopic Composition of the Samples" molality, molality, mol/kg RCH, Po, mol/kg solute of L20 n p17 p18 s-I s-I s-l solute of L20 n 0.110 0.024 0.070 7.54 512 KC1 3.89 0.023 4.74 Mi832 3.47 1.86 1.84 1.09
0.042 0.052 0.097 0.045
0.0 10 0.012 0.021 0.010
0.028 0.034 0.062 0.030
CaC1,
5.36 3.63 2.05 1.18 1.20 0.78 0.46
0.115 0.051 0.016 0.014 0.024 0.015 0.016
0.026 0.012 0.0039 0.0034 0.0059 0.0036 0.0039
LiCl
5.17 4.00 3.91 4.04 3.88 3.92 3.98 4.01 2.32 1.79 1.84 1.86 1.82 1.82 1.83 1.28 0.91
0.042 0.048 0.020 0.013 0.020 0.013
3.97 4.07 4.05 3.90 3.95 4.00 3.71 2.47 1.88 1.32 0.99 0.89
0.030 0.058 0.016 0.014 0.020 0.049 0.017
NaCl
-
5.60 3.61 3.63 2.87
367 241 240 197
0.073 0.033 0.0029 0.0024 0.0043 0.0026 0.0029
-
4.87 3.58 2.72 2.43 2.39 2.26 2.13
366 254 192 165 165 153 150
0.0 10 0.01 1 0.094 0.176 0.271 0.374 0.472 0.0049 0.0031 0.1 11 0.229 0.390 0.494 0.0050 0.0032
0.028 0.032 0.073 0.135 0.208 0.286 0.360 0.0036 0.0023 0.086 0.176 0.298 0.377 0.0036 0.0024
0.410 0.450 0.498 0.522 0.570 0.621 0.350 0.395 0.422 0.485 0.509 -
3.17 2.88 -
235 210
0.128
0.099
0.130 0.237 0.354 0.511 0.0141 0.0038 0.0034 0.0050 0.01 1 0.0040
0.100 0.182 0.271 0.390 0.010 0.0028 0.0025 0.0036 0.032 0.0029
0.353 0.400 0.431 0.485 0.539 -
-
-
-
-
-
2.18 2.08
154 151
2.43 -
192 -
2.39 2.19 2.18 2.06 2.03 2.01
0.019 0.021 0.017 0.015 0.022 0.017
5.73 4.35 3.90 3.99 3.99 4.07 3.89 4.06 2.34 1.03
0.091 0.038 0.067 -
0.058 0.025 0.014 0.152 0.239 0.286 0.390 0.063 0.029
0.436 0.517 0.576 0.591 0.662 -
3.32 2.78 2.73 2.41 2.18
270 229 212
0.099 0.044
0.020 0.009 0.017 0.197 0.3 12 0.374 0.511 0.022 0.010
CSCl
4.95 4.00 2.60 1.95 1.09
0.0045 0.0038 0.0046 0.0039 0.0075
0.001 1 0.0009 0.001 1 0.0010 0.0018
0.0008 0.0007 0.0008 0.0007 0.0013
-
1.78 1.78 1.80 1.82 1.86
140 137 137 135 137
KBr
4.88 3.87 2.60 1.92 0.90
0.0130 0.0099 0.0048 0.0093 0.0074
0.0032 0.0024 0.0012 0.0023 0.0018
0.0023 0.0018 0.0009 0.0017 0.001 3
-
1.72 1.72 1.76 1.77 1.84
136 132 132 132 135
4.98 3.83 2.84 1.91 0.69
0.0078 0.0095 0.0064 0.0096 0.0100
0.0019 0.0023 0.00 16 0.0023 0.0024
0.0014 0.0017 0.001 1 0.0017 0.0018
-
1.61 1.61 1.66 1.70 1.83
132 124 125 127 132
KF
KI
'An isotope fraction denoted by (-) means abundance (i.e., nnat= 0.156
Results and Discussion Corrections for Isotope Effects on the Relaxation Rates. Oxygen isotope effects on the reorientational motion of the solvent are expected to be proportional to the small effects on the viscosity of the solvent. The linear correction (10) in ref 13 will be applied. Relaxation rates corrected for this effect will be denoted by RF. Samples prepared for proton measurements were not enriched with deuterium; therefore the oxygen isotope correction is sufficient: RL =
(8)
Deuterium isotope effects on the reorientational motion of the solvent are assumed to be the same as in H 2 0 / D 2 0mixtures. The (17) D. E.Demco, P. van Hecke, and J. S.Waugh, J. Magn. Reson., 16, 461 (1974).
1.86 1.86 1.86 1.88 1.88 1.90 1.93
0.110 0.235 0.338 0.511 0.005 0.005 0.004 0.004 0.005 0.004
188 162 157 149 147 146
fluid thermostat using Fluorinert grade FC-43 (3M Co.). The magnetic field was locked with an external lock probe, using the F resonance in trifluoroacetic acid doped with copper acetate. TI measurements were obtained in duplicate at least, by the inversion recovery method with an estimated accuracy of 1%. FID were accumulated with a LSI-11 microcomputer, while the relative phase of A and the x / 2 pulses was alternated." One hundred data points were collected and fitted to a single exponential by a nonlinear least-squares procedure. The results are presented in Table 111.
0.254 0.281 0.314 0.348 0.382 -
RC,, s-I
-
-
-
s-'
-
-
2.43 2.32 -
s-I
3.96 3.89 4.00 4.10 4.00 3.67 2.94 1.94 1.56 0.93 0.58
-
173 164 -
-
GI
p18 0.0042 0.085 0.180 0.258 0.390 0.003 0.004 0.003 0.003 0.004 0.003
X
-
-
(~17)= " ~0.37 ~ X
p17 0.0057
-
-
-
143 141 139 140 140 140
-
181 156
( ~ 1 8 =) 2.04 ~ ~ ~X
observed D relaxation rate Rtbbsd and the " 0 relaxation rate RE (corrected for oxygen isotope effects) as a function of the deuterium mole fraction in these mixtures are given byI3
+ (0.423 f 0.004)n Rg(n) = (141.6 f 0.5) + (39.6 f l . l ) n
Rbsd(n)= (1.944 f 0.004)
(9a) (9b)
The intercept of eq 9a and 9b is just RL and R;, respectively, in eq 3, Le. the corrected values of pure water. Correction for D isotope effects was performed after correcting for oxygen isotope effects: RE, =
R8 =
1.944
1.944 (0.423~1)~
+
141.6 141.6 ( 3 9 . 6 ~ 1 ) ~ ~
+
Samples were enriched as little as possible, leading to small corrections on the order of 0.6%. Correlation times 7; determined by the D relaxation rates RL refer to HDO molecules. The transformation to correlation times for H 2 0 molecules isI3 r(HDO)H,o = (1.05 f 0.02)7(H20)H,o
(11)
where the molecular species involved is given within parentheses and the surrounding medium is denoted outside the parentheses.
1474 The Journal of Physical Chemistry, Vol. 90, No. 7. 1986
van der Maarel et al. 065-
Riisec"I 060-
055-
050OL5-
2 0-
OLO-
18-
035-
1 6-
030005
1 L-
00
2
L molality. m o l i k g H2O
6
Figure 1. Isotope effect corrected relaxation rates divided by the pure water values vs. the molality. For the different nuclei the following notations are used: D, filled symbols; I7O, open symbols. The symbols employed for the various salts are the following: CaCI,, 0; LiCI, 0; NaC1, A; and CsCI, V. Curves are drawn according to a quadratic polynomial fit.
TABLE I V Fitted Parameters of the Lines Displayed in Figure 3, along with the Values for Pure Water
solute LiCl NaCl KCI KF H20"
RkH,
molality
RCOH~
SKI
3.96 f 0.06 1.83 f 0.02 4.00 f 0.07 3.99 f 0.07 4.00 f 0.06
0.411 0.353 0.351 0.254 0.434 0.282
S-1
0.005 0.005 0.004 0.004 f 0.005 f 0.003 f f f f
0.44 f 0.02 0.32 f 0.02 0.37 f 0.01 0.26 f 0.01 0.44 f 0.02 0.275 f 0.009
Reference 13.
2.0/
& 1.8R'
055
035 045 '70 m o l e fraction
Figure 3. Oxygen isotope effect corrected proton relaxation rates vs. the 170mole fraction. Lines are drawn according to a linear least-squares fit. The line displayed for pure water is reported in ref 13.
1 2-
1
025
015
TABLE V Hydration Water Relaxation Rates and Correlation Times, Calculated with the Pure Water Values of the 0-H Bond Length and Deuterium Quadrupole Coupling Constant
1.61.L-
Li' 1.2 -
Na' K' F
4 1.8 4 4 4
4.11f0.07 3.9 f 0.1 3.10 0.06 1.74 f 0.05 3.81 f 0.07
*
0.64f0.04 0.51 f 0.09 0.48 f 0.03 0.23 f 0.03 0.64 f 0.02
4.1f0.2 3.9 f 0.2 3.1 f 0.2 1.8 f 0.1 3.9 f 0.2
4.6f0.3 3.7 f 0.7 3.5 f 0.2 1.7 f 0.2 4.7 f 0.3
1.0-
a value of 0.98 A seems preferable. Using this value in the calculations in ref 13 one obtains
0.8 -
TO
0 0
2 L m o l a l i t y , mol / k g
6
H20
Figure 2. As in Figure 1. The symbols employed for the various salts are the following: KF, 0; KCI, 0 ;and KI, A.
As a first approximation this solute isotope effect on hydratiqn water is expected to be the same as on pure water. For 170the analogous effect will be neglected. Deuterium correlation times corrected for this effect will be denoted by &H20). Corrected relaxation rates are presented in Table 111. Deuterium and axygen-17 relaxation rates RC,, and Po,respectively, divided by the values of pure water RL and R t , respectively, are displayed in Figures 1 and 2 as a function of the molality. Large effects are observed when small and/or highly charged ions are in solution. Determination of the Coupling Constants. To obtain the effective correlation times it is necessary to know the interaction constants. With the intramolecular dipolar H-170 interaction contribution to the H relaxation rate of pure water it is possible to obtain the reorientational correlation time TO. A value of 1.71 ps was re orted earlier, but the 0-H distance was assumed to be 0.958 , Le. the gas value. Also with this correlation time and the quadrupole relaxation rates of pure water, values of quadrupole coupling constants have been obtained.13 Because recent neutron diffraction experiments show a somewhat larger 0 - H bond
1
= 1.95 f 0.08 ps
(?)' (?)'
(12)
= 252 f 6 kHz
(13a)
= 8.0 f 0.2 MHz
(13b)
D
0
with 7, = 0.135 and 70 = 0.75 (gas values). The coupling constants of the water molecules in the hydration shell may be discussed now. To this end the contribution of I7O to the proton relaxation was determined experimentally to allow a comparison with the deuterium relaxation rate. Proton relaxation rates RL are displayed in Figure 3 as a function of the oxygen-17 mole fraction. In view of the observed linearity one has Rh = R&H + (p17)RbH
(14)
in which R&H and RgH denote the H-H and H-I7O, respectively, dipolar interaction contribution to the H relaxation rate. The (18) G. W. Neilson and J. E. Enderby, Proc. R.SOC.London, Ser. A 390, 353 (1983). (19) J. G.Powles, Mol. Phys., 42,757 (1981). (20) W. E.Thiessen and A. H. Narten, J . Chem. Phys., 77,2656 (1982).
The Journal of Physical Chemistry, Vol. 90, No. 7, 1986 1475
Water Dynamics in Aqueous Electrolyte Solutions TABLE VI: Some Calculated Quantities of Hydration Water 76H9
ion
m
4
ps
5.0 f 0.8 1.8 3 f 1 Na+ 4 3.7 f 0.6 K+ 4 1.6 f 0.4 F 4 5.3 f 0.6
Li'
6H,
A
(e2qQlh)gl (e2qQlh)61 (e2qQlh)?, (e2qQ/h)?,
0.993 f 0.009 0.91 f 0.07 0.89 f 0.08 0.97 f 0.03 0.992 f 0.009 0.92 f 0.07 0.90 f 0.08 0.97 f 0.02 1.001 f 0.006 0.85 f 0.05 0.82 f 0.06
TABLE VII: Linear Coefficients, of Rcx/Rox in the Molality, m
Bx,of the Polynomial Expansion
solute
Bi7
BO
MgC4 CaCI, LiCl NaCl
0.372 f 0.009 0.14 f 0.02 0.102 f 0.008 0.05 f 0.01 -0.027 f 0.003 -0.041 0.003 -0.056 0.005 -0.077 f 0.005 0.06 f 0.02
0.23 f 0.04 0.08 f 0.01 0.072 f 0.009 0.022 f 0.008 -0.021 f 0.001 -0.030 f 0.005 -0.051 f 0.005 -0.080 f 0.007 0.08 0.01
KCI CSCl
*
*
KBr intermolecular contribution to RbH in pure water, obtained with KI a formula derived by Hubbard accounting for translational difKF fusion,21appears to be 0.0054 s-'.I3 In salt solutions this intermolecular contribution is unknown but will be taken to be 2% of pronounced in the hydration shell of F. Neutron scattering the total H-I7O dipolar interaction contribution, as it is in pure experiments have not detected such a change of the 0-H bond water. Relaxation rates RhH and R& resulting from the fit of length.'* A recent MD study of the structure of the Li+ hydration eq 14 to the data of Figure 3 are presented in Table IV. shell shows a similar increase of the 0-H bond length, using the Hydration water relaxation rates R$ and RgH calculated accentral force potential.24 The 0-H distance does not change cording to eq 3 with RC, and the intramolecular part of R& are significantly in the hydration shell of 4 m K+. It is not possible presented in Table V. Hydration water effective correlation times to study a wide range of concentrations, because the experimental obtained with the pure water values of the 0-H bond length and error increases a t lower concentrations. the D quadrupole coupling constant are also presented in Table The effect on the D quadrupole coupling constant is somewhat V. These correlation times, T$(H,O) and T&, of 4 m LiC1, NaC1, more pronounced but so is the estimated error. The D coupling and K F solutions differ just outside experimental error. This is constant in the hydration shell of 4 m Li', Na+, or F is somewhat not the case when the solute is 4 m KCl. In case of a 1.8 m LiCl smaller than the pure water value. In the hydration shell of K+ solution the experimental accuracy is not good enough to justify the coupling constant does not change significantly. The I7O a discrimination. Another choice of hydration number or a small coupling constant will also be affected by the ionic charge, but influence of the counterion (Cl-) cannot abolish these effects. in the liquid state, under extreme narrowing conditions, there Because the principal directions of the interaction tensors coincide appears to be no experimental procedure to obtain the effective and the value of the D asymmetry parameter is small, these 170correlation time separately. Solid-state N Q R provides a correlation times are expected to be equal within experimental method to determine the quadrupole coupling constants in crystal error as was discussed in the Introduction. The difference between hydrates and different forms of ice. Poplett observed a correlation the effective correlation times 7g(H20) and T& of water molebetween the I7O and D coupling constants for crystal hydrates, cules within the hydration shell of 4 m Li+, Na+, and F indicate ice, and water vapor:25 that the 0-H distance and/or the D quadrupole coupling constant have changed with respect to pure water. = A change of the quadrupole coupling constant from pure water to hydration water can be caused by a polarization of the water 0 molecule by the ionic charge which also induces a change in geometry of the molecule. Ab initio quantum chemical calcula(38.214 f l.2!33)( - (1650 f 302) kHz (16) tions show the main factor determining the D quadrupole coupling D constant to be the 0 - D bond length. An increase in bond angle The pure water values, eq 13a and 13b, also satisfy this relation. was found to produce only a small reduction, while the dependence As the coupling constants were calculated by assuming isotropy, on bond length is nearly linear and independent of whether the molecule is isolated or incorporated in dimer or i ~ e . ~For~ a , ~ ~ this confirms the isotropy in pure water. Relation 16 will be used to determine the hydration water I7O coupling constant. The small change of 0-H distance this dependence is approximately hydration water 170quadruple coupling constant divided by the linear: value of pure water, obtained according to this procedure for 4 m salt solutions, is also presented in Table VI. A study of water dynamics by interpreting the ratio of the I7O and D relaxation rates is not sensitive to a particular choice of the coupling constants, because as a consequence of relation 16 in which r t H denotes the pure water 0-H bond length. The the 170coupling constant is nearly proportional to the D coupling constant a is obtained by interpolating the gas and ice value of constant, as was indicated in the case of 4 m Li+, Na+, and F. the coupling constant. A value of 1805.8 kHz/A will be used. Therefore the pure water values of the coupling constants will be With eq 4, 5, and 15, the hydration water relaxation rates Rg and used. RgHand the condition, r$(H20) = 7&, together with the pure Relaxation Rates of Hydration Water at Infinite Dilution. The water values of the D quadrupole coupling constant and 0-H relaxation rates in the limit of infinite dilution are now estimated distance it is possible to derive the hydration water values of the to obtain the relaxation behavior of an isolated hydration complex. 0-H distance, the D quadrupole coupling constant, and the efThe procedure proposed by Hertz9 will be used. Hertz proposed fective hydration water correlation time. These values are a polynomial expansion of the relaxation rates in the molality of presented in Table VI. A deuterium isotope effect on the 0-H the solution. Fitted quadratic polynomials are displayed in Figures bond length has been neglected. 1 and 2; they describe the relaxation behavior well. Under the The 0-H distance in pure water is assumed to be 0.98 A. In assumptions discussed in the Introduction the hydration water this first experimental N M R determination of the influence of relaxation rates of the complex at infinite dilution are related to ions on the bond length of water in the hydration shell the results the linear coefficients B, of the polynomial expansion according are marginally outside the estimated error margin. The 0-H bond to of hydration water molecules of 4 m Li+, Na+, and F is lengthened due to the polarization by the ionic charge. This effect is most R:(m-+O)/R! = -B, 55.5 + 1 (17) n*
*
(?)
F)
~
~
~~
(21) P. S. Hubbard, Phys. Reu., 131, 275 (1963). (22) E. R. Davidson and K. Morokuma, Chem. Phys. Lett., 111,7(1984). (23) P. L. Cummins, G. B. Bacskay, N. S. Hush, B. Halle, and S. Engstrom, J . Chem. Phys., 82, 2002 (1985).
(24) P. Bopp, I. Okado, H. Ohtaki, and K. Heinzinger, Z.Nazurforsch.
A., 40, 116 (1985). (25) I. J. F. Poplett, J . Magn. Reson., 50, 397 (1982).
1476 The Journal of Physical Chemistry, Vol. 90, No. 7, 1986
van der Maarel et al.
P 0
100
200
3 00
LOO
5 00
Y
Figure 5. Theoretically expected orientation of the anion-oxygen axis. Polar coordinates: @ = 90°, 8.
Figure 4. Hydration water relaxation rates of an infinitely diluted complex divided by the pure water values as a function of the electric field intensity on the surface of the ion. For the different nuclei the following notations are used: I'O, filled symbols; D, open symbols. Squares denote
the anions. while for cations circles are used. Linear coefficients 8,are presented in Table VII, relative hydration water relaxation rates R:(m-+O)/R: are displayed in Figure 4 as a function of the relative electric field strength q/(ge) on the surface of the isolated ion. A linear correlation is found between R:(m+O)/ R: and the electric field strength according to R:/Rt
= (0.32 f 0.13)
R$/R$ = (0.52 f 0.04)
+ (0.89 f 0.06)q/(+e)
+ (0.57 f 0.02)q/(r2e) + (3.5 f 0.2)q/(+e) f 0.11) + (4.0 f 0.3)q/($e)
(18a)
Figure 6. Theoretically expected orientation of the cation-oxygen axis. Polar coordinates: @ = Oo, 0.
(1 8b)
R b / R i = (-0.42 f 0.08)
(18c)
R,/R$ = (-0.51
(18d)
The relative trend will be equal if one chooses another value of n* in eq 17 or a certain influence of the counterion. The importance of the polarity of the ionic field (i.e., the cationic and anionic lines in Figure 4 do not coincide) shows that the interaction of a water molecule with its surrounding medium is not only the ion point charge-dipole interaction. The quantity R:(m-+O)/R: reflects the relative change of the quadrupole coupling constant and the correlation time of infinitely diluted hydration water with respect to pure water according to
""\I I
\
R r-
Figure 7. Definition of the orientation of the D,, axis of the diffusion tensor with respect to the molecular frame, characterized by the angles a and p.
As the relative effect of ionic field strength on the D and " 0 coupling constants is nearly equal, the different behavior of the D and "0 relaxation rates cannot be explained by a change of the coupling constants. Therefore the data displayed in Figure 4 directly indicate the effectiveness of the relaxation mechanisms, i.e. the effective correlation times. As depicted in Figure 4 the anionic I7O and D hydration water effective correlation times coincide within experimental accuracy. The motion within the hydration shell is not expected to be isotropic, because (especially in case of F) several experimental methods indicate a significant anion-water interaction. Neutron scattering pair correlation functions indicate that the anion-water axis coincides with the 0-H axis.26 Recent N M R experiments ~ orientation as given by suggest a symmetric ~ r i e n t a t i o n . ~The M D simulations depends strongly on the interaction functions.6sz8 However, from these results, the anion and hydration water molecule are expected to be coplanar as indicated in Figure 5. Now, if the Dllaxis of the diffusion tensor is positioned in the molecular plane of the water molecule, i.e. (3 = 0 in Figure 7, the (26) J. E. Enderby and G. W . Neilson, Rep. Prog. Phys., 44, 38 (1981). (27) K. J. Miiller and H. G. Hertz, 2. Phys. Chem. (Frankfurt am Main), 140, 31 (1984). (28) P . Bopp, W. Dietz, and K. Heinzinger, Z . Naturforsch. A, 34, 1424 (1979).
effective correlation time for D relaxation should be longer than the 170correlation time. This conflicts with the observed equality of the correlation times. If we assume anisotropic axially symmetric diffusion for the hydrated water molecules it turns out that equal correlation times for D and I7O are obtained within experimental accuracy for a set of values for a and @ depending on the ratio Dl,/D,. N o solution is obtained, however, for @ 5 45'. Therefore, the DlI axis does not lie within the molecular plane. In case of cation hydration water the D and 170effective correlation times differ outside experimental error. The difference in the effect of the cationic field intensity on the relaxation rates of D and I7O directly confirms the anisotropy of the environment of the water molecules in the hydration shell of the cations. The experimental cationic effective hydration water correlation times will now be discussed in terms of the orientation of the diffusion tensor and the amount of anisotropy. The orientation of the principal axis system of the diffusion tensor with respect to the molecular frame is characterized by the angles a and @ defined in Figure 7. Theoretical considerat i o n ~M , ~D~results,6 and the neutron scattering ion-water pair correlation functionsz6strongly suggest that the interaction of water molecules and cations is symmetrical with respect to the plane containing the bisectrix of the H 2 0 bond angle and the cation. (29) E. J. W. Verwey, R e d . Trau. Chim., Pays-Bas., 61, 127 (1942).
The Journal of Physical Chemistry, Vol. 90, No. 7, 1986 1477
Water Dynamics in Aqueous Electrolyte Solutions
DII,DI
LOO0
11010sec-11
\
3000-
2000-
?
CL*
p Figure 8. Reorientational diffusion constants, Dlland D,, of cationic hydration water of an infinitely diluted complex as a function of the angle 6, a = 90°. The pure water constant is denoted by Do. 000
2000
'
LOO0
'
6000
'
0
0
1 2
L m o l a l i t y . m o l l k g H20
Figure 10. Cationic hydration water relaxation rates divided by the values of pure water vs. the molality. CI- was used as a reference. For the different nuclei the following notations are used: D, open symbols; I7O, filled symbols. The symbols employed for the different cations are the following: Mgz+,0; Ca2+,0;Li+, A;Na+, V; and Cs", 0 . Curves are drawn as an aid to the eye. Dashed curves denote I7O relaxation, while for D relaxation rates solid curves are used.
000
2000
'
LOO0
6000
'
p
Figure 9. The degree of anisotropy,D N / D , ,of cationic hydration water of an infinitely diluted complex vs. the angle 6. It will thereore be assumed that the symmetry axis of the diffusion tensor is confined to the bisector plane of the water molecule (Le., (Y = 90') leaving only the angle 0 to be determined from the effective correlation times for the I7O and D relaxation rates in the hydration shell. From the two simultaneous equations for the effective correlation times 7: and 7fD(H20)it is possible to calculate the elements Dlland D , of the diffusion tensor for a given value of 0. It was checked that small changes in the coupling constants as determined at 4 m salt solutions do not significantly change the results discussed below. As the coupling constants for hydration water in infinitely diluted salt solutions are unknown, the bulk water constants have been used. It turns out that physically acceptable solutions are only available in the range 0 C p 5 30' for Mg2+, Ca2+,Na+, and Li+, while in the case of the large cations Cs+ and K+ p 2 40' is found. For Mg2+,Na+, and Cs+ the possible solutions are displayed in Figure 8. As the angle is defined with respect to the molecular plane of the water molecule, each value of /3 implies two possible orientations of DIIwith respect to the vector connecting the cation and the oxygen nucleus of the water molecule. From the symmetry of the cation-water interaction it seems reasonable to choose the DIIorientation coincident with the direction of the electric field of the cation, which is nearly parallel to the cation-oxygen axis, as displayed in Figure 6. Thus the angle p characterizes the orientation of the water molecule with respect to the cation and it equals 0 in Figure 6. Recent N M R experiments show the angle 0 of the water orientation within the lithium hydration shell of a 0.2 m LiCl solution to be in the range 0-30° if an outer first hydration shell contribution to the 7Li relaxation rate is accounted for.30 It may be seen from Figure 8 that around the highly charged and small cations the water molecules are oriented with a small angle between their bisectrix and the ionic field. Concurrently these structure forming ions slow down the molecular motion of the neighboring water molecules and induce a certain amount of (30) R. Mazitov, K. J. Miiller, and H. G. Hertz, Z. Phys. Chem. (Frankfurt am Main), 140, 55 (1984).
anisotropy in their motion, as depicted in Figure 9. For large ions such as Cs+ and to a lesser degree K+ the dipole-ionic field interaction is relatively less important. In this context it is of interest to note that, if the angle 0 increases to 5 2 O , the hydration water molecule may form hydrogen bonds with surrounding water molecules. It is seen in Figure 9 that, for p = 5 2 O , the structure-breaking ions still induce some motional anisotropy. From these N M R data regarding the cationic influence on the water molecules at infinite dilution it is seen that this influence on the dynamical behavior is limited. If we assume bulk water to reorient isotropically a distinct but moderate anisotropy is induced even by Mg2+ and Li+. Effects of Electrolyte Concentration. After this discussion of the influence of ions with a common counterion on water molecules in the hydration shell of these ions at infinite dilution of the salts, attention may now directed to the concentration dependence of the water nuclear relaxation rates as depicted in Figures 1 and 2. As the dependence of the relaxation rates on the salt molality is nonlinear it must be concluded that the rates in the hydration shell, in bulk water or in both phases, depend on the molality. Now it should be noted that the ratio of the D and I7Orelaxation rates can only change due to a change of the diffusion tensor, because a sufficient change in the ratio of the coupling constants for these nuclei is not to be expected. From Figures 1 and 2 it is seen that the ratio RE/R: is clearly concentration dependent. In view of the moderate anisotropy of the dynamical behavior of the hydration water molecules it is improbable that significant anisotropy will be induced outside the first hydration shell. Therefore, the concentration dependence of RC,,RC,, and RC,/RCowill be assigned to the first hydration shell. Indeed neutron scattering pair correlation functions for water nuclei around NiZ+and Li+ show a concentration dependence of the orientation within the first hydration she11F6 Relative hydration water relaxation rates R:/R: calculated with the model discussed in the Introduction are displayed in Figures 10 and 11. The concentration-dependent dynamic behavior of the cation hydration shell may now be discussed. According to the rotation diffusion model used here, the effective D correlation time is longer than the effective 170correlation time if the angle /3 5 30' (Figure 7). If the orientational angle /3 of the diffusion tensor increases to >, 40° the 170correlation time becomes the larger of the two. According to eq 19 the relative relaxation rates R:/R: should
1478 The Journal of Physical Chemistry, Vol. 90, No. 7, 1986
van der Maarel et al.
Figure 11. As in figure 10, but for anionic hydration water. Kt was used as a reference. The symbols employed for the different anions are the following: F, 0; C1-, 0;and I-, A.
surface. This trend continues with K+ and Cs+ where, even at infinite dilution, one finds p 2 40'. Rotation diffusion constants have not been calculated as a function of concentration, because the exact values of (3 are of course unknown. The degree of anisotropy will be quite moderate anyway, as was already found for infinite dilution. The concentration dependences of the relative D and "0 anion hydration water relaxation rates, as displayed in Figure 1 1, differ outside experimental error. As discussed in the preceding section the anion and hydration water molecule are expected to be coplanar, as displayed in Figure 5. If the Dl,axis of the diffusion tensor is positioned in the molecular plane, i.e. p = 0, the effective D correlation time has to be longer than the I7O correlation time. The relative anion hydration water relaxation rates, as displayed in Figure 11, however, show a longer I7O effective correlation time in the whole concentration range and for all anions studied (see eq 19). The motion within the hydration shell is clearly anisotropic, but the values for a and p depend on the ratio Dll/D,. No solutions are, however, obtained for p 5 45'. The D,, axis is not positioned in the molecular plane. As in case of cationic hydration water the degree of anisotropy is moderate. The anisotropy of the motion within the anionic hydration shell at higher concentrations supports the expectation of anisotropic motion in the limit of infinite dilution.
behave in exactly the same way as the effective correlation times. Thus it is concluded from Figure 10 that the angle p in the hydration shell of Mg2+should remain below -30' in the range of concentrations studied. This contrasts with the behavior around Ni2+ where 8 reaches a value of 42' at 1.5 m.26 Alternatively this might indicate that in the Mg2+ hydration shell D,, is not completely coincident with the direction of the ionic electric field (i.e., p # 8). The results for Ca2+and Li+ show that the relative D and 170relaxation rates cross in the 4 m range. Here is seen to increase from values /3 5 30' at low concentration to /3 2 40' at high concentration, definitely indicating a change in the hydration water properties as a function of the salt concentration. For Na+ ions the increase of /3 to the region /3 2 40' occurs at 0 relatively low concentration (1-2 m) while in the limit m p is found to be less than 30°. In the hydration shell of the larger ions, K+ and Cs+, p 2 40' is found in the whole concentration range. Thus one arrives at the following view of the orientation of water molecules in the first hydration shell of cations if /3 equals 8. In the case of small highly charged cations (Mg2+),there is a clear orientation of the water molecule in the direction of the ion ( p 5 30') even at high concentrations. For CaZ+,Li', and Na+ a perturbation of the orientation at infinite dilution occurs at concentrations that increase with the electric field at the ionic
Conclusions Magnetic relaxation rates of the nuclei of the water molecules yield structural and dynamical information about hydration water that is complementary to neutron diffraction results. The influence of the ions on the 0-H bond length and on the D quadrupole coupling constant is found to be significant but moderate for the ions investigated here. The orientational mobility of the water molecules decreases in the proximity of the structure-making ions and it increases near the structure-breaking ions. A moderate anisotropy is observed in the reorientational motion. For the cation hydration water the major axis of the molecular diffusion tensor may consistently be taken to lie in the bisectrix plane of the water molecule. For the anionic hydration water this axis does not lie in the plane of the water molecule. The increasing amount of structural information from neutron diffraction and the application of N M R may allow the investigation of more detailed dynamical models in the near future. In such a model the approximate description in terms of free anisotropic rotational diffusion should probably be extended to include interaction potentials. Registry No. MgCI,, 7786-30-3; KCI, 7447-40-7; CsCI, 7647-1 7-8; LiCI, 7447-41-8; NaCI, 7647-14-5; CaCI,, 10043-52-4; KI, 7681-1 1-0; KBr, 7758-02-3; H,O, 7732-18-5; " 0 , 13968-48-4; D,, 7782-39-0.
2
RX -
R;
1
0
I
'
I
2 L m o l a l i t y , m o l l k g H20
-
